In industrial measurements technology (especially in connection with the regulating and monitoring of automated processes), in order to ascertain characteristic measurement variables of flowing media (for example liquids and/or gasses) in a process line (for example a pipeline), measuring systems are often used which—by means of a vibration-type measuring transducer and connected measuring device electronics (usually situated in a separate electronics housing) with a driver- and evaluating circuit—induce reaction forces (for example Coriolis forces) in flowing media and produce, derived from these forces, a measurement signal correspondingly representing the at least one measured variable (for example a mass flow rate, a density, a viscosity or other process parameter).
Measuring systems of this sort (often formed by means of an in-line measuring device of a compact design with an integrated measuring transducer, for instance a Coriolis mass flow meter) have long been known and have proved themselves in industrial use. Examples of measuring systems with a vibration-type measuring transducer as well as individual components thereof, are described, for example, in EP-A 317 340, EP-A 848 234, the JP-A 8-136311, the JP-A 9-015015, US-A 2007/0119264, US-A 2007/0119265, US-A 2007/0151370, US-A 2007/0151371, US-A 2007/0186685, US-A 2008/0034893, US-A 2008/0141789, U.S. Pat. Nos. 4,738,144, 4,777,833, 4,777,833, 4,801,897, 4,823,614, 4,879,911, 5,009,109, 5,024,104, 5,050,439, 5,291,792, 5,301,557, 5,398,554, 5,734,112, 5,476,013, 5,531,126, 5,602,345, 5,691,485, 5,796,010, 5,731,527, 5,796,011, 5,796,012, 5,804,741, 5,869,770, 5,945,609, 5,979,246, 6,047,457, 6,092,429, 6,073,495, 6,311,136, 6,223,605, 6,330,832, 6,397,685, 6,557,422, 6,651,513, 6,666,098, 6,691,583, 6,776,052, 6,799,476, 6,840,109, 6,883,387, 6,920,798, 7,017,424, 7,040,179, 7,073,396, 7,077,014, 7,080,564, 7,200,503, 7,216,550, 7,299,699, 7,318,356, 7,360,451, 7,392,709, WO-A 00 14 485, WO-A 01 02 816, WO-A 07/130024, WO-A 08/013545, WO-A 08/07 7574, WO-A 99/28708, WO-A 99 40 394 or WO-A 96/02812.
Each of the therein illustrated measuring transducers comprises at least one essentially straight or at least one curved (for example U or V-shaped) measuring tube (made, for example, of stainless steel, titanium, zirconium or tantalum) used for the conveyance of a medium (in certain cases, an extremely cold or extremely hot medium). For the purpose of generating oscillation forms in part influenced by the medium flowing through, during operation of the measuring system, the at least one measuring tube (extending with an oscillatory length between an inlet side, first measuring tube end and an outlet side, second measuring tube end) is caused to vibrate (especially in a bending oscillation mode) about an imagined oscillation axis imaginarily connecting the two ends.
For exciting oscillations of the at least one measuring tube, vibration-type measuring transducers consequently further exhibit an exciter mechanism, which is driven by an electric driver signal (e.g. a regulated electrical current) generated and correspondingly conditioned by the driver circuit of the measuring device electronics; and which—by means of at least one electromechanical (especially electrodynamic) oscillation exciter through which a current flows during operation and which acts practically directly on the measuring tube—excites mechanical oscillations (for example bending oscillations) in the at least one measuring tube, and, in this respect, converts the electrical power fed into the measuring transducer by the measuring electronics into mechanical movements. Moreover, measuring transducers of this sort include a sensor mechanism having at least two (especially electrodynamic and/or equally-embodied) oscillation sensors for at least point-specific registering of inlet-side and outlet-side oscillations (especially those in Coriolis mode) of the at least one measuring tube and for the production of electrical sensor signals, which serve as primary signals for the measuring transducer, and which are influenced by the process parameter to be registered, for instance the mass flow rate, the totaled mass flow or the density.
In the case of measuring transducers with curved (e.g. U, V or Ω-shaped) measuring tubes, for the oscillation form to be excited—the so-called driving or wanted mode—that particular oscillation form is normally chosen in which the measuring tube, in the case of a lowest natural resonance frequency, at least partially swings about an imaginary longitudinal axis of the measuring transducer in a pendulum-like fashion in the manner of a cantilever secured at one end; whereby Coriolis forces dependant on the mass flow are induced in the medium flowing through. These, in turn, lead to the fact that, in the case of a curved measuring tube and thus pendulum-like cantilever oscillations, bending oscillations of a frequency equal to the excited oscillations of the wanted mode are superimposed on the latter, according to at least a second, likewise natural oscillation form, the so-called Coriolis mode. In the case of measuring transducers with a curved measuring tube, these cantilever oscillations forced by the Coriolis forces in Coriolis mode normally correspond to that eigenoscillation form, in which the measuring tube also performs rotary oscillations about an imagined rotary oscillation axis aligned perpendicular to the longitudinal axis. Conversely, in the case of measuring transducers with straight measuring tubes, for the purpose of producing mass-flow-dependant Coriolis forces, a particular wanted mode is often chosen in which the measuring tube at least partially performs bending oscillations essentially in a single imaginary plane of oscillation, so that the oscillations in Coriolis mode are accordingly formed as bending oscillations coplanar to the wanted mode oscillations with an oscillation frequency equal to them. The two ends of the measuring tube are thus in this respect defined by those two particular oscillation nodes that are common to the wanted and Coriolis modes. In the case of a curved measuring tube, the oscillatory length therefore corresponds practically to a laid out straight length of an essentially freely oscillating section of the respective measuring tube, extending between the two oscillation nodes.
Due to the superimposition of the wanted and Coriolis modes, the inlet-side and outlet-side oscillations (which are registered by means of the sensor arrangement) of the vibrating measuring tube also exhibit a measurable phase difference, which is dependent on the mass flow. Normally, during operation, measuring tubes of this sort of measuring transducer (e.g. mass flow meters used in Coriolis) are excited to an instantaneous natural resonance frequency of the oscillation form chosen for the wanted mode, especially at an oscillation amplitude regulated to be constant. As this resonance frequency is, among other things, particularly dependent on the instantaneous density of the medium, the density of the medium, in addition to the mass flow, can be measured by means of a commercially available mass-flow meter. Furthermore, as shown in U.S. Pat. Nos. 6,651,513 or 7,080,564, it is also possible by means of a vibration-type measuring transducer to directly measure the viscosity of a medium flowing through, for example on the basis of an excitation power necessary for the excitation of the oscillations.
In the case of measuring transducers with two measuring tubes, the two measuring tubes are normally incorporated into the process line via an inlet-side distributor element extending between the measuring tubes and an inlet-side connecting flange, as well as via an outlet-side distributor element extending between the measuring tubes and an outlet-side connecting flange. In the case of measuring transducers with a single measuring tube, the latter normally communicates with the process line via an essentially straight connecting tube segment opening into the inlet-side, as well as via an essentially straight connecting tube segment opening into the outlet-side. Furthermore, each of the illustrated measuring transducers with a single measuring tube comprises at least a one-piece or multi-part embodied (for example tube, box or plate-shaped) counteroscillator, which is coupled to the measuring tube to form on the inlet side a first coupling zone and to form on the outlet side a second coupling zone; and which, during operation, essentially rests or oscillates opposite-equally, that is with equal frequency and opposite phase. The inner part of the measuring transducer, formed by means of the measuring tube and the counteroscillator, is normally held alone by means of the two connecting tube pieces, via which the measuring tube communicates with the process line during operation, in a measuring transducer housing, especially in a manner which makes possible the oscillations of the inner part relative to the measuring tube. In the case of the measuring transducers with a single, essentially straight measuring tube (as shown, for example U.S. Pat. Nos. 5,291,792, 5,796,010, 5,945,609, 7,077,014, US-A 2007/0119264, WO-A 01 02 816 or WO-A 99 40 394), the measuring tube and the counteroscillator are essentially arranged coaxially to each other, as is typical with traditional measuring transducers. In the case of those measuring transducers of the aforementioned type available on the market, the counteroscillator is normally also essentially tube-shaped and formed as an essentially straight, hollow cylinder, which is arranged in the measuring transducer in such a way that the measuring tube is at least partially jacketed by the counteroscillator. Comparatively inexpensive types of steel, for example structural steel or free-machining steel, are usually used as materials for such counteroscillators, even or especially in the case of the use of titanium, tantalum or zirconium for the measuring tube.
The exciter mechanism of measuring transducers of the type discussed here normally exhibits at least one electrodynamic oscillation exciter and/or at least one oscillation exciter that acts differentially upon the at least one measuring tube (and, in given cases, on the present counteroscillator or the present second measuring tube), while the sensor arrangement comprises an inlet-side, usually also electrodynamics oscillation sensor as well as at least one in essence equally embodied outlet-side oscillation sensor. Such electrodynamic and/or differential oscillation exciters of the vibration-type measuring transducers available on the market are normally formed by means of a magnet coil though at least part of which an electrical current flows (in the case of measurement transducers with one measuring tube and a counteroscillator coupled thereto, the magnet coil is usually fixed to counteroscillator), as well as by means of an elongated—especially rod-shaped—permanent magnet, which 1) serves as an armature, which 2) interacts with (and especially plunges into) the magnet coil, and which 3) is correspondingly fixed to the measuring tube to be moved. The permanent magnet and the magnet coil which is to serve as the exciter coil are normally arranged in such a way that they extend essentially coaxial to each other. Additionally, in traditional measuring transducers, the exciter mechanism is normally designed and placed in the measuring transducer in such a way that it in essence acts centrally on the at least one measuring tube. In such a case, the oscillation exciter—and, in this respect, the exciter mechanism—is normally fixed to the measuring tube at least pointwise along an imagined central peripheral line of the latter, as shown in the case of the measuring transducers proposed in U.S. Pat. Nos. 5,796,010, 6,840,109, 7,077,014 or 7,017,424.
In the case of most vibration-type measuring transducers available on the market, the oscillation sensors of the sensor arrangement are, as previously suggested, essentially of an equal construction to the oscillation exciter, at least insofar as they function according to the same principle of action. Consequently, the oscillation sensors of such a sensor arrangement are also in each case usually formed by means of an permanently magnetic armature (which is affixed to the measuring tube and which delivers a magnetic field), as well as by means of a coil which 1) interacts with the armature, which 2) is permeated by its magnetic field, which 3) at least at time supplied with an induced measurement voltage, and 4) which is normally affixed to the counteroscillator insofar as it is present, and otherwise affixed to one of the measuring tubes. Each of the aforementioned coils is additionally connected by means of at least one pair of electrical connecting lines to the mentioned operating and evaluation electronics. These lines most often run via the shortest route possible from the coils, over the counteroscillator, to the transducer housing.
The measuring device electronics of inline measuring devices of the aforementioned type normally available on the market most often exhibit a microcomputer—for example formed by means of a digital signal processor (DSP)—which delivers digital measured vales in real time. This microcomputer usually includes, in addition to at least one corresponding processor and associated circuit components (e.g. an A/D converter and a D/A converter), corresponding volatile and non-volatile data memories as well, for storing digital measurement and operation data ascertained internally or externally transmitted to the respective in-line measuring device, for instance for storing those chemical or physical properties relevant (for instance serving as a reference) to the measurement of the medium to be measured. In addition to the microcomputer and the driver circuit (which makes the operation of the measuring transducer possible), the measuring device electronics normally further exhibit an input circuit, which implements for the microcomputer the conditioning of the measurement signals delivered by the measuring transducer, and which (forming the aforementioned measurement and evaluating circuit of the measuring device electronics) is correspondingly interconnected with the microcomputer. Based upon the measurement signals delivered by the measuring transducer and/or upon the driver signals (which drive the measuring transducer) delivered by the measuring device electronics, the microcomputer ascertains the desired primary measured values—for instance an instantaneous mass flow rate of the medium flowing through the measuring transducer, and/or a totaled mass flow, which corresponds to a mass of the mediums which has, overall, flowed through the measuring transducer during a predetermined period of time—and provides these in real time.
Since conventional measuring systems of the type discussed are normally embodied as independent measuring devices which are to be incorporated (for example via a 2-wire or 4-wire line) into a superordinated electronic data processing system (for example a system which controls a filling process and/or one formed by means of a programmable logic controller (PLC)), the measuring device electronics of modern inline measuring devices of the type discussed in each case also exhibit a corresponding communication circuit, which makes possible the transmission and reception of measurement or operation data. This communication circuit occurs, for example, in the form of a digital output of the sort established in industrial measurements and automation technology, in the form of an established 4-20 mA electrical current signal output, in the form of a bus interface conforming to NAMUR recommendation NE43:1994 and/or to the PROFIBUS standard IEC 61158 or in the form of another interface circuit conforming to an industry standard. Additionally provided in the measuring device electronics is a supply circuit, which assures the supplying of the in-line measuring device with energy and which obtains the necessary energy from an internal energy storer and/or from the electronic data processing system via a 4-wire line or by means of a 2-wire line, the latter line for example embodied as a 4-20 mA current loop with measuring-device-side load modulation.
As can be drawn, among other things, from the previously mentioned EP-A 848 234 or WO-A 96/02812, in the case of a measuring system formed by means of a vibration-type measuring transducer, for the achievement of the desired—and no less expected—high accuracy of measurement, a particular meaning is to be ascribed to the positioning of the primary oscillation sensor relative to the chosen oscillation nodes of the oscillations of the measuring tube or measuring tubes excited for the purpose of measurement of the primary measured variable, mass flow. According to EP-A 848 234 or WO-A 99/28708, a lower sensitivity to disturbance variables (for instance from external vibrations)—and in this respect a high accuracy of measurement for the measuring system in question—would additionally be reachable by placing the oscillation sensors in each case as near as possible to the oscillation nodes of the previously mentioned wanted mode, whereby its share of the oscillations registered by means of the sensor arrangement—and in this respect of the respective primary signal—is kept as low as possible. In the case of conventional measuring systems of the type discussed which are available on the market, especially in the case of those solely with oscillation exciters acting centrally on the measuring tube, for the purpose of achieving as high a sensitivity as possibly to the primary measured variables (especially the mass flow or mass flow rate), with, simultaneously, a lowest possible sensitivity to possible disturbance variables, as well as a sufficiently high signal-to-noise ratio for the primary signal in the area of the excited oscillation frequency, the oscillation sensors of the sensor mechanism are placed in the measuring transducer in such a way that a measuring length of the measuring transducer corresponding to a length of a region of the measuring tube extending between the first oscillation sensor and second oscillation sensor amounts to more that 65% of the oscillatory length. In the case of a curved measuring tube, the measuring length then corresponds to a laid out straight length of the essentially freely oscillating section of the respective measuring tube extending between the two oscillation sensors.
Investigations of various measuring systems of the type discussed here have, however, shown that a disadvantage of positioning the oscillation sensors in the aforementioned manner exists in the fact that, for achieving an oscillation amplitude of 10-15 μm at the site of the oscillation sensors sufficient for the desired high accuracy of measurement, maximum oscillation amplitudes relatively large for the respective measuring system of about 30 μm at the center of the measuring tube are necessary for the excited oscillations in the wanted mode. In association therewith, a relatively high electrical power, in some cases amounting to far greater than 100 mW, must be converted in the exciter mechanism, this being especially true in the case of low mass flow rates.
Not least of all in the case of in-line measuring devices (as for example is proposed in the previously mentioned U.S. Pat. Nos. 6,799,476 or 7,200,503) whose measuring device electronics are, during operation, to be connected with a superordinated data processing system solely by means of a 2-wire connection (for example a 4-20 mA current loop) serving both the measured data and the energy transfer, both the continually available as well as the maximum allowable electrical power is, as is known, limited (depending on the voltage supply used or allowed from a technical safety point of view), to about 40-150 mW or to 1 W, depending on the particular case, so that, among other things, sufficient energy is not always available during operation to allow for the provision of the signal-to-noise ratio or the noise separation actually required. For the case mentioned—in which the in-line measuring device is, by means of a 4-20 mA current loop, both externally supplied with energy and also provided with the measured values by adjusting the electrical current level flowing through the current loop proportionally thereto—the less electrical power becomes available to the measuring system overall (and in this respect the less electrical exciter power becomes available to the measuring transducer), the more power such would actually be necessary for the desired high measurement of accuracy. Moreover, an additional disadvantage of a higher maximum oscillation amplitude exists in the fact that a multiplied degree of unwanted, disturbing vibrations can be provoked, and as a result, the measurement system's overall susceptibility to disturbance is correspondingly increased.